Bone marrow derived professional antigen presenting cells (pAPC) are required to initiate antiviral cytotoxic T lymphocytes (CTL) responses. However, when a virus cannot infect the relevant pAPC, CTL responses are still generated because pAPC can acquire antigens from other infected cells and present them to CTL in a process known as cross-presentation. The overall objective of the parent project 1R01AI048849-01A1 is to define the mechanisms of MHC class I antigen cross-presentation that occur in viral infections and result in successful CTL priming. During the course of our experiments, we have found that some types of pAPC but not others can cross present antigens from vaccinia virus infected cells. The overall goal of this new project is to take advantage of the functional difference between these pAPC to identify proteins that may participate in cross presentation of antigens from infected cells. In Aim 1, we will use cDNA arrays to identify candidate genes. In Aim 2, we will use small interfering RNA and retroviral vectors to silence or express those genes and test in a functional assay whether they are involved in cross presentation.